1. Field of the Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a ballistic system including an impedance barrier. Yet more specifically, the present invention relates to a perforating gun system whose shaped charges are held in a medium, and where a gap is formed within the medium between each adjacent shaped charge.
2. Description of Related Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore, and the casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems 40 typically comprise one or more perforating guns 42 strung together, these strings of guns 42 can sometimes surpass a thousand feet of perforating length. Included with the perforating guns are shaped charges 44 that typically include a housing, a liner, an initiator, and a quantity of high explosive inserted between the liner and the housing. A detonating cord 46 attached to each shaped charge sequentially actuates the initiator within each shaped charge. The perforating systems are generally lowered into a wellbore on wireline or tubing 48 where the initiation of the perforating gun detonation is transmitted through the wireline or tubing 48. Firing heads 50 are typically included on the perforating guns for receiving the transmitted detonation signal from the surface 52 and in turn igniting the detonation cord 46.
When the high explosive within the shaped charge 44 is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a “jet” 54. The jet penetrates the casing, the cement and a quantity of the formation 56 thereby forming a conduit 58 by which the hydrocarbons entrained within the formation may be drained into the wellbore for production at the wellbore surface.
In addition to the perforating jet formed by detonation of the shaped charges, the charges also produce shock waves that emanate into the formation and along the perforating gun 42. These shock waves can be transmitted onto other shaped charges prior to or during their detonation and interfere with the trajectory of the perforating jet 54. This jet interference can in turn create curved perforations and reduce the overall depth of the perforations 58. Curved and shorter perforations present an undesired condition since this can reduce the production capability of hydrocarbon bearing formations. Therefore, there exists a need for an apparatus and method capable of impeding the shock and/or vibration transmitted between shaped charges.